Trunk transmission network

ABSTRACT

In a trunk transmission network for transmitting information signals between nodes via paths, flexible path operation is achieved by setting up paths between source nodes and destination nodes after pre-classifying paths into a higher service class in which any loss of information occurring in that path is restored, and a lower service class which permits loss of information to occur in the path. The flexible operation is further achieved by arranging for each node, when it acts as a source node, to recognize the service class of the information signal it is sending to a destination node, and to select a path corresponding to that service class.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is based on and claims priority from JapaneseApplication No. 8-225492 filed Aug. 27, 1996, No. 8-237169 filed Sep. 2,1996, and No. 8-326944 filed Dec. 6, 1996, the contents of each of whichis incorporated hereinto by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention can be utilized for trunk transmission systems inwhich communication involves setting up paths semi-permanently onphysical transmission lines, and it is suited for use in synchronousdigital hierarchy (SDH) networks. A path is defined as a single anddirect connection between a node-pair, for example, concatenated VirtualContainer (VC) in SDH format, and may include Virtual Paths (VPs) havingthe same source-destination nodes in the trunk network.

2. Description of Related Art

In recent years, ultrahigh-speed trunk transmission networks thatutilize the broadband nature of optical fiber have been introduced. Inparticular, as disclosed, a 10 Gbit/s transmission system has beenintroduced in trunk network links, as disclosed in:

Ref.1: Y. Kobayashi, Y. Sato, K. Aida, K. Hagimoto and K. Nakagawa,“SDH-Based 10 Gbit/s Optical Transmission System”, Proc. IEEE GLOBECOM94 (San Francisco, Calif.), pp.1166-1170, 1994

Meanwhile, asynchronous transfer mode (ATM), which supports diverseservices, has been recommended by the ITU-T and the ATM Forum as thesignal processing scheme for network nodes. In the ATM layer, which ispositioned between the physical layer and the application layer, signalsare processed in cell units. However, problems are encountered if theprocessing of signals carried in trunk network links at speeds in excessof 10 Gbit/s is carried out entirely in cell units. As many as severalthousand virtual paths (VPs) have to be processed at each node, whichmeans that large-scale node circuitry and more complex networkmanagement is required, as disclosed in:

Ref.2: S. Matsuoka, N. Kawase, Y. Yamabayashi and Y. Kobayashi,“Classified Path Restoration Scheme With Hitless Protection Switchingfor Large-Capacity Trunk Transmission Networks”, IEEE GLOBECOM 95,p.941-945, 1995

Given this situation, the present inventors have that although thesignal processing employed in the ATM layer can be utilized for servicenodes, trunk network node processing functions such as path setup andrestoration will be carried out in large-capacity direct-connected pathunits at the physical layer. These large-capacity direct-connected pathscan have a variety of capacities, and the management of path networkscan be simplified by processing in large-capacity direct-connected pathunits. It is also considered that time division multiplexing (TDM) willbe used at the physical layer for multiplexing. In the presentspecification, it will generally be assumed that Synchronous DigitalHierarchy (SDH) is being used.

Meanwhile, high reliability and survivability are required in networkswith ultrahigh-capacity links, as disclosed in:

Ref.3: T.-H. Wu, “Fibre Network Service Survivability”, Artech House,Boston and London, 1992

In an ultrahigh-speed network, a failure in one fiber can have adverseeffects on several thousands of users.

Self-healing functions are therefore being studied and introduced.Self-healing is a high-speed restoration function for network failures,and the best-known example to have been introduced is the SONET(Synchronous Optical Network) ring network in which path or lineswitches are provided. A self-healing ring network has the advantages ofsimpler equipment configuration and higher reliability. Problems ofdelay and the like mean that a multiple-ring configuration combining aplurality of rings is a promising approach to the design of trunknetworks. However, a multiple-ring network with a self-healing functionhas not yet been achieved, and path setup functions such as routing andslot allocation have not yet been perfected.

Network supervision and control will now be explained. The TMN(Telecommunication Management Network) model has been standardized, andits architecture is shown in FIG. 1. In this architecture, a networkelement NE provided at each node is connected to a packet transfernetwork DCN (data communication network) via a message converter moduleMCM (or a mediation device MD, not shown), and an operating system OpSis connected to this packet transfer network DCN. FIG. 1 also shows aworkstation WS for using operating system OpS. Each network element NEhas a control section which exchanges control signals with the operatingsystem OpS, and transfers supervisory and control information to theOpS, via the message converter module MCM (or a mediation device MD) andthe packet network DCN.

However, as transmission link-capacity of the networks becomes larger,the cost of the operating system OpS in the model shown in FIG. 1, andin particular software development cost, becomes higher than that of thenetwork elements NE, thus raising overall network costs. Moreover, witha centralized control network of the sort shown in the FIG. 1, if thesystem goes down at the control node, this leads to the entire networkgoing down.

Distributed control has therefore been much studied. In distributedcontrol, network control is performed in distributed fashion at eachnetwork node. FIG. 2 shows a distributed management network architecturein a single-ring network. With this architecture, a small-scaleoperating system OpS is provided at each network element NE. Distributedcontrol of this sort is disclosed in, for example:

Ref.4: I. Cidon, I. Gopal, M. Kaplan and S. Kutten, “A DistributedControl Architecture of High-Speed Networks”, IEEE Transactions onCommunications, Vol.43, No.5, pp.1950-1960, 1995

A distributed control network requires only a small-scale operatingsystem provided in each network element, and gives higher reliability inrelation to node failure than a centralized control network with severalcontrol nodes, as disclosed in:

Ref.5: A. E. Baratz, J. P. Gray, P. E. Green, Jr., J. M. Jaffe and D. P.Pozefsky, “SNA Networks of Small Systems”, IEEE Journal on SelectedAreas in Communications, Vol.SAC-3, No.3, pp.416-426, 1985

Further advantages are that a separate control network such as DCN isnot needed, the network database memory held by each node can be reducedin size, and faster control is possible.

It is anticipated that in the future there will be many different kindsof multimedia services and that each kind will require different signalquality or reliability. Trunk networks will therefore have to operateand administrate multiplexed paths for each service in accordance with adiverse range of quality requirements, and do so at low cost.

However, conventional network technology handles the quality andreliability of all paths in the same manner. Consequently, the qualityand reliability of a network has previously been dictated by the pathwhich has the highest requirements, with the result that overall networkcost has been high. An approach which was studied as a way of overcomingthis problem, namely, to provide for different QoS (Quality of Service)classes by means of a logically configured virtual channel handler (VCH)interconnection network layer rather than at the VP layer, is describedin:

Ref.6: E. Oki and N. Yamanaka, “An Optimum Logical-Design Scheme forFlexible Multi-QoS ATM Networks Guaranteeing Reliability”, IEICE Trans.Commun., E78-B, No.7, pp.1016-1024, 1995

However, this proposed scheme still required a high-quality VP networkand lacked flexibility at the path operating level.

It is considered that future multimedia networks will requireflexibility at the path level as well. In other words, such networkswill simultaneously contain paths where high cost is acceptable but lossof even a single bit is not acceptable, and other paths where somedeterioration of quality or reliability is acceptable but cost should bekept low.

It is an object of the present invention to provide a solution to thisproblem and to achieve flexibility of path operation. It is a furtherobject of the present invention to provide a concrete implementation, ina multiple-ring architecture under a distributed control environment, ofthe operation of paths that have been classified in accordance withtheir self-healing survivability.

SUMMARY OF THE INVENTION

The present invention provides a trunk transmission network having aplurality of nodes connected via physical transmission lines, wherein aplurality of paths for transmitting information signals are set up onthese physical transmission lines among the plurality of nodes. Forinformation signal being transmitted from one of the plurality of nodes(a source node) to another of the plurality of nodes (a destinationnode), each path connects the source node and the destination nodeeither directly or via other nodes. This trunk transmission network ischaracterized in that paths between source and destination node pairsare set up on the basis of a pre-classification into higher serviceclass paths in which any loss of information occurring in the path isrestored, and lower service class paths in which loss of information inthe path is permitted. Each node includes means which, when that node isa source node, recognizes the service class of the information signal tobe sent to the destination node and selects a path corresponding to thisservice class.

The higher service class is preferably further divided into a highestclass (hereinafter class A) and a middle class (hereinafter class B).Class A paths employ complete diversity routing: namely, a plurality ofdifferent routes are set up for each class A path. Class B paths can bere-routed around the location of a failure when a failure has occurredin a portion of the route traversed by the path. The lowest serviceclass path (hereinafter class C) is preferably a path which is notalternatively routed when a failure has occurred on the path.

By dividing paths between nodes of interest into three classes accordingto their restoration performance in the event of a failure, thetransport functions required by service nodes can be secured at the pathlevel without configuring redundant sections, thereby providing a trunktransmission network that can economize on transmission facilities.Furthermore, by managing the network using just three types oflarge-capacity paths, the number of paths that have to be managed in thenetwork can be reduced, and hence the burden on the operating system canbe eased.

Each of at least some of the plurality of nodes preferably has adistributed path setup means which sets up paths prior to transmissionof an information signal by using a control channel to exchange controlsignals with other nodes. In this case, the distributed path setup meansselects a route, in accordance with the required service class, fromamong the plurality of routes which can connect the source anddestination nodes, and then sets up a path along the selected route.Path setup methods can be broadly divided into two types. In the firsttype, a node which wishes to transmit data takes itself as the sourcenode and provisionally determines routes on the basis of networkconfiguration information given in a manual. It then secures bandwidthby sending a control signal to all the nodes on the route up to thetarget receiving node. In the second type of path setup method, a sourcenode uses a token protocol to send a packet to a destination node, andany intermediate nodes place a stamp in the packet indicative of whetheror not the required bandwidth can be secured. This procedure enables theroute to be determined and the necessary bandwidth to be secured.

The physical transmission lines are in the form of a plurality of ringnetworks connected together, each ring network comprising two or morenodes connected in a ring. Each ring network is connected to anotherring network by means of some of the network nodes acting as bridgenodes. The distributed path setup means preferably includes means which,for a class A path, sets up two paths in mutually opposite directions,i.e., clockwise or counterclockwise, around each ring network throughwhich the class A path passes, and which, for a class B path and a classC path, sets up a path in one direction around each ring network.

By restricting a trunk transmission network to a ring topology, settingthe direction of routes is restricted to either clockwise orcounter-clockwise, routing in the normal state and re-routing for pathrestoration, etc. after a failure can be simplified, the hardware andalgorithms required for route compilation can be reduced in scale, andan economical trunk transmission network can be obtained. In addition,by arranging a plurality of ring networks in a plane and connectingthese ring networks to each other using two or more nodes, highreliability, survivability and economy can be secured for large-scaletrunk transmission networks, and at the same time expandability can beimproved.

If the second of the methods described above is utilized for path setup,a token ring protocol constructed on a data communication channel (DCC)in the section overhead (SOH) embedded in the signal is preferably usedfor communication between nodes and for securing bandwidth, thesefunctions being required for route determination under distributedcontrol.

In other words, the means for path setup preferably comprises meanswhich, when the node in question is a source node, gets a token which iscirculating around the ring network to which that node belongs, and thensends path setup request packets in two mutually opposite directions;means which, when the node in question is a bridge node, transfers apath setup request packet that has arrived in one direction to the nextring network in the same direction; and means which, when the node inquestion is a destination node and the packets received from the twodirections request setup of a class A path, sends back a response tothese packets in two mutually opposite directions, and when the packetsreceived from the two directions request setup of a class B or a class Cpath, sends back a response to one of these packets in one directiononly.

For self-healing, each node preferably has means for hitlessly selectingthe better quality route of the class A path including two routes forwhich that node is the destination node. The path setup means preferablyalso includes means for automatically restoring a class B path byre-routing in the event of a failure. This restoration means preferablyincludes means which utilizes the second setup method described above toloop back a token contained in the aforementioned control channel when anode has detected a failure in an adjacent link or node.

In other words, three classes of paths, namely class A, class B andclass C are provided, and these offer three levels of reliability interms of path restoration performance. Class A paths are accommodated bytwo different routes obtained by route-bifurcation at the source node,and these two routes are hitlessly switched at the destination node,with the result that when a failure occurs a class A path can berestored without the loss of even a single bit of information. Adetailed description of hitless switching is given in:

Ref.7: N. Kawase, et al., “Hitless Frame Switching Scheme for SDHNetworks”, Trans. IEICE B-I (in Japanese), Vol.J78-B-I, No.12,pp.764-772, 1995

Class B paths, which have the next highest reliability, are restored byre-setup of the path by means of the same method as used for theoriginal path setup. In the case of a class C path, the signal is notreconnected until maintenance of physical equipment has been completed.Apart from class B paths, the mechanism of self-healing is approximatelythe same as the one disclosed in Ref.2.

Each ring network is connected to another ring network via two or morebridge nodes. At least one node of any of the ring networks includesmeans for transmitting, in one direction (i.e., clockwise orcounter-clockwise) of the ring network to which that node belongs, anode information collecting packet for collecting information relatingto the arrangement and operating state of the nodes in that ring networkand in the other ring networks, and means which terminates a nodeinformation collecting packet which has returned to the node whichoriginally transmitted it, and which stores the information collected bythat packet. Each node of each ring network includes means which writesits node ID and states in a received node information collecting packetand transfers the packet to the next node. Each node used as a bridgenode includes, in addition to this node information collecting packetwriting and transfer means, means for temporarily storing a nodeinformation collecting packet received from one of the two ring networksmutually connected by the bridge node in question; means which, when aright to transmit to the other of the two ring networks has beenreceived, transfers to this other ring network the node informationcollecting packet stored in the aforementioned temporary storage means;means which deletes the node information collecting packet stored in thetemporary storage means if no transmitting right has been obtained and anode information collecting packet from another bridge node has beenreceived; and means for terminating a node information collecting packetwhich has returned to the bridge node which originally transferred it,and for write inhibiting that packet and returning it to the originalring network.

Each node and bridge node preferably includes means which, if itreceives the same node information collecting packet within a predefinedtime, deletes this packet. A source node preferably also has means whichdistributes, to at least the bridge nodes of the plurality of ringnetworks, and if required to each node, the information collected by anode information collecting packet . Each bridge node preferablyincludes means which, on the basis of the information distributed fromthis distributing means, places a restriction on the setup of paths viathat bridge node.

The inventions of Japanese Laid-Open Patent Applications Hei-3-276937and Hei-3-217140 disclose providing differences in path priority.According to the former, the quality of a high priority path isguaranteed by sacrificing a healthy low priority path in the event of afailure in the high priority path. As opposed to this, in the presentinvention, both high priority paths and low priority paths includealternative paths and are set up in advance, and the setup of thesepaths is not changed when there is a failure. Accordingly, a lowpriority path is never sacrificed for a high priority path. Furthermore,Japanese Laid-Open Patent Application Hei-3-276937 discloses sharedswitching being carried out for purposes of path restoration, but thereis no route duplication as in the highest class paths in the presentinvention. Japanese Laid-Open Patent Application Hei-3-276937 alsodiscloses master nodes which perform centralized control. As opposed tothis, the present invention performs distributed control.

Japanese Laid-Open Patent Application Hei-3-217140 relates to packetnetworks in which data transmission takes place only when data has beengenerated. Furthermore, it allocates one of two transmission paths tourgent data, but does not provide degrees of priority for data transferwhen a failure has occurred. Moreover, master nodes perform centralizedcontrol of the data transmission paths. The present invention isentirely different from this.

BRIEF DESCRIPTION OF THE DRAWINGS

Other objects and advantages of this invention will become apparent fromthe detailed description of the preferred embodiments in conjunctionwith the drawings, in which:

FIG. 1 is a prior art example, showing management network architecturebased on the TMN model for a single ring network.

FIG. 2 is a prior art example, showing distributed management networkarchitecture for a single ring network.

FIG. 3 is a conceptual diagram of a trunk transmission-network accordingto a first embodiment of the present invention.

FIG. 4 is a block diagram of a node.

FIG. 5 shows the routing of a class A path.

FIG. 6 shows the routing of a class B path.

FIG. 7 shows the routing of a class C path.

FIG. 8 shows an arrangement of nodes.

FIG. 9 shows another arrangement of nodes.

FIG. 10 shows an example of a routing algorithm.

FIG. 11 shows an example of routing a class A path.

FIG. 12 shows an example of routing a class B path.

FIG. 13 shows a second embodiment of the present invention.

FIG. 14 serves to explain path restoration methods when a link failurehas occurred.

FIG. 15 shows the frame format for a token ring.

FIG. 16 is a block diagram of a node.

FIG. 17 shows the overall control flow for path setup, involving eachtype of node.

FIG. 18 shows the control flow at a source node.

FIG. 19 shows the control flow at an intermediate node.

FIG. 20 shows the control flow at a bridge node.

FIG. 21 shows the control flow at a destination node.

FIG. 22 shows the relation between path connection request frequency andmean connection delay.

FIG. 23 shows an example of a configuration for hitless switching.

FIG. 24 shows the bit error rate improvement obtained by hitlessswitching.

FIG. 25 shows the situation where a failure has occurred in the j-thring network containing nodes A, B, C, D and Z.

FIG. 26 shows the relation between node position and the number of pathswhich have to be restored, on the assumption that all failed paths areclass B.

FIG. 27 shows schematically how a token is transferred between all ofthe nodes by loop-back.

FIG. 28 shows restoration ratios obtained by calculation.

FIG. 29 shows a third embodiment of the present invention.

FIG. 30 shows the processing flow for a source node which wants tocollect information relating to node arrangements and conditions.

FIG. 31 shows the processing flow at an intermediate node.

FIG. 32 shows the processing flow at a bridge node.

FIG. 33 shows the packet processing flow in the processing shown in FIG.32.

FIG. 34 shows an example of path setup.

FIG. 35 shows the processing flow for routing by a bridge node.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 3 and FIG. 4 show a first embodiment of the present invention, withFIG. 3 being a conceptual diagram of a trunk transmission network andFIG. 4 being a block diagram of a node.

The FIG. 3 trunk transmission network contains nodes A to C and physicaltransmission lines connecting these nodes A to C. A path fortransmitting information signals from a source node to a destinationnode is set up via nodes A to C and these physical transmission lines,both the source node and the destination node being one of nodes A to C.

A plurality of paths of different transmission quality are establishedin advance from the source node to the destination node by means ofmultiplexed frame terminator 13 (FIG. 4), control interface 21, pathdata collector 10, path router 11 and cross connection 14. Theinformation signal contains a service class identifier ID, and thesource node uses service class identification 12 to recognize thisidentifier, and path selector 15 to select one of the plurality of pathsin accordance with this service class.

There are three service classes. Class A transmits information aftersetting up two routes. Under normal conditions, class B transmitsinformation by means of one route, but when a failure has occurred inthis route, it transmits information after setting up a route whichdetours around the location of the failure. Class C transmitsinformation by means of one route and does nothing to make up forinformation loss due to occurrence of a failure.

Next, the operation of this embodiment will be explained with referenceto FIG. 5 to FIG. 7. FIG. 5 shows the routing of a class A path, FIG. 6shows the routing of a class B path, and FIG. 7 shows the routing of aclass C path. FIG. 5 shows the routing of a class A path from node A tonode D. At source node A, after the information to be transmitted hasbeen split into two, it is transmitted by means of route 1-1 and route1-2, in which the transmission directions are respectivelycounter-clockwise via node B and node C, and clockwise via node F andnode E. The destination node detects a failure on the basis of the stateof the transmission in route 1-1 and route 1-2, and selects theinformation in the route that is normal. Several methods have previouslybeen considered for implementing hitless switching using this completediversity routing. However, the use of hitless switching based on paritydetection as disclosed in the aforementioned Ref.7 results not only inhitless switching when a transmission line failure has occurred, butalso in a high quality path with improved transmission error rate innormal operation, so this method is considered to be optimum.

FIG. 6 shows the routing of a class B path. Under normal operatingconditions, a complete diversity routing configuration as in a class Apath is not adopted for this path. However, when a failure occurs, asearch is made for a route which detours around the failed section andthe path is restored by changing to this other route.

FIG. 7 shows the routing of a class C path. For this path, the completediversity routing configuration of the sort employed for a class A pathis not adopted, nor is a search made for an alternative route when afailure occurs, as in the case of a class B path. Consequently, the pathis restored by repairing the failed equipment, for example the physicaltransmission line, the transmission equipment, and so forth. By thusproviding paths with different self-healing levels, it is possible toconstruct a trunk transmission network in which important informationcan always reach its destination, and in which information where thechief consideration is economy can easily be transmitted cheaply.

As shown in FIG. 5 to FIG. 7, the transmission of information from nodeA to node D requires just three paths, namely, the class A, class B andclass C paths described above. Administration of a trunk transmissionnetwork in which the number of nodes is assumed to be N thereforerequires management of a maximum of 3 N(N−1)/2 paths.

FIG. 8 shows an arrangement of nodes. Ring networks generally have theadvantages of good survivability and the ability to provide simplerrouting than other topologies. In this specification, trunk transmissionnetworks having a plurality of rings will be discussed. This is becausea plurality of rings is required when a large-scale trunk transmissionnetwork to cover an entire country is to be constructed on the basis ofrings. FIG. 8 shows the fundamental node arrangement, in which ring αand ring β are connected by the two nodes B and G. A node that connectsrings in this way is called a bridge node. Two bridge nodes are used notonly because this makes it possible to restore a path by detouringaround a failed link when a single-link failure has occurred in a trunktransmission network, but also because it makes it possible to restoreinformation (i.e., a path) between any two nodes (other than the failednode) in the event of a node failure. By using a configuration of thissort it is also possible to configure a route which implements a class Apath between any two nodes, i.e., two diversity-routed routes that donot intersect at any intermediate node. Moreover, because each node hasfrom two to three adjacent nodes, only a restricted number of routeshave to be set up at a given node, and therefore it is possible use asimple algorithm to route a path in the normal state or to re-route apath in the event of a failure. Note that the link between node B andnode G in FIG. 8 is shared by ring α and ring β.

FIG. 9 is another arrangement of nodes, showing an example in which thenodes have been extended and the number of rings has been increased tothree. Here it is essential to connect adjacent rings by means of twonodes, and ring α is connected to ring γ by nodes G and H, while ring βis connected to ring γ by nodes F and G.

As illustrated in FIG. 1, according to the path management methoddisclosed in the prior art, an operating system OpS held the sparecapacities of all the nodes in the form of a database, and after havingretrieved optimum routes, sent connection requests to network elementsNE (i.e., nodes) via message converter modules MCM (or mediation devicesMD).

On the other hand, the embodiment shown in FIG. 3 does not use anoperating system located in a separate device from nodes A to C.Instead, it performs path management using main signal processor 22,path management section 20 and controller 21, all of which are providedin each node A to C. Controller 21 exchanges control signals with othernodes. Main signal processor 22 manages various functions of the mainsignal system, such as multiplexed frame signal termination,multiplexing and demultiplexing, path routing and release, low-speedframe signal termination, and inter-office and intra-officetransmissions. In path management section 20, path data collector 10shown in FIG. 4 obtains the capacities of the links at the local node(i.e., the capacities of the paths that could be connected) and theconnection status of any paths. Path router 11 gives instructions andthe like regarding routing and release of paths to main signal processor22 when a path connection or release request occurs. In addition,service class identification 12 identifies the service class to whichthe information signal to be transmitted belongs, and on the basis ofthis identification instructs main signal processor 22 to transfer theinformation signal to the relevant path. Controller 21 is used as ameans for communicating with path management section 20 of other nodes.At each node, control information is extracted from the received signalby main signal processor 22 and is handed over to controller 21.Controller 21 extracts information relating to paths from the controlinformation and transfers it to path management section 20. In the caseof transmitting, information is transferred in the opposite direction.The actual communication is carried out by packet transfer. It does notmatter whether these packets use a data communication channel (DCC) orthe like in the overhead provided in the multiplexed frame signal, orwhether they use a channel in another signal frame based on wavelengthdivision multiplexing.

FIG. 10 shows an example of a routing algorithm. This routing algorithmutilizes the first type of path setup method described above. Namely,when there is a path setup request, without using a centralizedoperating system, the node at which the path setup request has occurredsuccessively sets up a route to the destination node while confirmingspare capacity by accessing each node individually. Exchanges betweennodes involve sending and receiving control information betweenrespective path management sections 20 shown in FIG. 3. In FIG. 10, itis supposed that node A has requested path setup to node E. Becausetarget node E belongs to ring β rather than to node A's ring α, node Aselects the route via bridge node C, since this is the shortest route toring P. Accordingly, as the first stage of the setup process, node Aconfirms whether a path (1) can be set up to node B. After sparecapacity has been confirmed and the route has been provisionallyregistered, node A performs a similar confirmation at node B withrespect to node C via path (2). After the route has been provisionallyregistered, bridge node C selects the route via bridge node D, thisbeing the shortest route to node E, and performs a similar confirmationwith respect to bridge node D (path (3)). After the route has beenprovisionally registered, bridge node D performs a similar confirmationwith respect to node E, which is the target node (path (4)). Node Econfirms that it itself is the target node and that spare capacity fromthe requesting node to the target node can be guaranteed, and aftersetting up the connection (path (5)), forwards an instruction to set upup/down channels, in the reverse order to the confirmation orderdescribed above (path (6)). After receiving the setup instruction fromnode E, bridge node D forwards the setup instruction to bridge node C(path (7)). Setup is completed in similar manner at node A, therebycompleting the routing process (path (8) and path (9)).

FIG. 11 shows an example of routing a class A path. Node A, at whichsetup of a class A path has been requested, requests the setup of twocompletely diverse routes, namely clockwise route I andcounter-clockwise route II. Node A uses the clockwise route to transmitto node F the information that node J is the target node (path (1)).After spare capacity has been confirmed and the route has beenprovisionally registered, node F transmits the information from node Ato node E (path (2)). Node E performs similar processing (path (3)).Bridge node D determines that for the clockwise route it itself is thebridge node to ring β and accordingly selects the clockwise route inring β and transmits the information to node J (path (4)). Node Jconfirms that it itself is the target node and that spare capacity fromthe requesting node to the target node can be guaranteed, and aftersetting up the connection (path (5)), forwards a setup instruction inthe reverse order to the confirmation order described above. Node A alsouses a counter-clockwise route to transmit to node B the informationthat node J is the target node (path (6)). After spare capacity has beenconfirmed and the route has been provisionally registered, node Btransmits the information from node A to bridge node C (path (7)).Bridge node C determines that for the counter-clockwise route it itselfis the bridge node to ring P and accordingly selects thecounter-clockwise route in ring β and transmits the information to nodeG (path (8)). After spare capacity has been confirmed and the route hasbeen provisionally registered, node G transmits the information fromnode A to node H (path (9). Similar processing is performed by nodes Hand I as well (path (10) and path (11)). Node J confirms that it itselfis the target node and that spare capacity from the requesting node tothe target node can be guaranteed, and after setting up the connection(path (12)), forwards a setup instruction in the reverse order to theconfirmation order described above. The setup of a path including twodiverse routes is completed by this sequence of processing steps.

FIG. 12 shows an example of routing a class B path. Because a class Apath includes two routes giving complete routing diversity, in the eventof a failure the path can be restored by selecting the healthy, fullycomplete route at the receiving node. Because a class C path is restoredby an equipment repair, nothing happens automatically when a failureoccurs. On the other hand, in the case of a class B path, in the eventof a failure the path has to be restored by searching for a new andalternative route. In the present invention, the restoration of a classB path is performed using the routing algorithm shown in FIG. 10. FIG.12 shows the situation where a failure has occurred between node B andbridge node C. Node A detects that a failure has occurred in ring α andthat route B-1 from itself to node J has been broken. Node A thereforechanges route B-1 (ring α counter-clockwise followed by ring βclockwise) to route B-2 (ring a clockwise followed by ring β clockwise).The new route is set up by node A accessing nodes F and E and bridgenode D in the manner explained in FIG. 10 or FIG. 1. Note that for classB path setup, spare capacity confirmation is necessary at everycommunication with the other nodes.

In the foregoing embodiment, the packet transfer protocol can be aconnection-oriented one such as X.25, or a connectionless one such asthe IP protocol, and communication between nodes is performed on aone-to-one basis. Furthermore, in the foregoing embodiment the networkconfiguration data has to be given in advance in a manual.

Next, the second method for distributed setup of paths will beexplained.

FIG. 13 shows a second embodiment of the present invention. This secondembodiment comprises ring network 41 in which a plurality of nodes 31 to35 are connected in a ring by transmission lines, and ring network 42 inwhich nodes 36 to 39 and nodes 33 and 34 are similarly connected in aring by transmission lines. These two ring networks 41 and 42 areconnected to one another by making nodes 33 and 34, which belong to bothnetworks, into bridge nodes. Ring network 42 is also connected toanother ring network by making nodes 37 and 38 into bridge nodes. Eachof nodes 31-39 is capable of setting up a path to other nodes by using acontrol channel to send control signals to other nodes. A path is theunit of information transfer.

This embodiment employs high-speed TDM for data transmission and uses atoken ring protocol in order to perform path setup by means ofdistributed operation. This token ring protocol is constructed on theDCC in the section overhead or on a channel in another signal framebased on wavelength division multiplexing. The reason for using a tokenring protocol is that the connections in trunk networks are relativelylong-lasting and are not changed too frequently, so that connectiondelay due to token access is not a problem in practice.

Paths are classified into three different grades according toreliability and quality, namely class A, class B and class C. For aclass A path, the highest grade of path, two paths are set up inmutually opposite directions around each ring network through which theclass A path passes. FIG. 13 shows a class A path between nodes 31 and36. One of the two paths including the class A path (the clockwise one)is connected to bridge node 33 via node 32, dropped by the add-dropmultiplexer (ADM) in this node 33, traverses switch SW, and is connectedto ring network 42 by being added from the add-drop multiplexer ADM onthe ring network 42 side of switch SW. The path is then connected toreceiving node 36. The second of the two paths including the class Apath (the counter-clockwise path) is connected to bridge node 34 vianode 35, and is then connected to ring network 42 in the mannerdescribed above with reference to node 33. The path is then connected todestination node 36 via nodes 39, 38 and 37. In the case of a class Apath of this sort, which comprises two separate and diverse paths, theone with the better quality is hitlessly selected by means of hitlessswitching unit 51 at the destination node. A hitless switching unit 51is provided at each of nodes 31 to 39, but for present purposes only thehitless switching unit provided at node 36 is shown.

For class B and class C paths, a path is set up in one direction aroundeach ring network through which the class B or class C path passes. FIG.13 shows a class B path between nodes 39 and 35.

In a multiple-ring topology of this sort, the number of working pathsthat can be accommodated in each ring, assuming full mesh connectivityfor the paths, is expressed by the following equation: $\begin{matrix}{A = {\frac{N^{2} - 1}{8} + {\frac{1}{2}\left( {n - j} \right)\left( {j - 1} \right)\left( {N - 1} \right)^{2}} + {\frac{1}{2}\left( {N - 1} \right)^{2}{\max \left\lbrack {{n - j},{j - 1}} \right\rbrack}}}} & (1)\end{matrix}$

where N is the number of nodes in a ring, n is the number of rings, andj is the ring number counted from the left. In Equation 1, the firstterm is the number of intra-ring paths, the second term is the number ofpaths passing through the j-th ring, and the third term is the number ofpaths from the j-th ring to another ring. It will of course be seen thatthe number of paths accommodated in a ring is largest for ringspositioned in the middle of the topology.

FIG. 14 serves to explain path restoration methods when a link failureoccurs. For the class A path, restoration is achieved by selecting thenon-failed route at hitless switching unit 51 at the receiving node. Forthe class B path, restoration is achieved by re-routing within ringnetwork 41 only.

FIG. 15 shows the frame format for a token ring constructed on the DCC.In this frame, FC stands for frame control which includes delimiter andtoken, D-ID is the destination node ID, S-ID is the source node ID, andC is a packet type identification field indicating whether the packet isa routing packet, a response packet. The frame also includes a datafield and a frame check sequence FCS. The data field contains a numberof different areas. The grade area indicates the class of the path (thisis equivalent to the service class identifier in the first embodiment).The c/cc area shows the direction of packet flow (clockwise orcounter-clockwise). The capacity of the path for which there is aconnection request is written in the path capacity area. Theintermediate node area holds the flags set by each intermediate nodebetween the source and destination nodes. The bridge (drop) ID areacontains an identifier of either the bridge node or the destinationnode. Finally, there is an area showing whether or not a path spans aring.

The present invention is not restricted to the token protocol shown inFIG. 15 and can likewise be implemented by the IBM Token-Ring protocolor by the Loop 1 Protocol of Mitsubishi Electric. Moreover, if thecontrol scheme uses a token protocol, it can be implemented using eithera multi-token scheme or an early token release scheme. Although it willbe assumed that each ring is a bi-directional 2-fiber ring, thisinvention can be embodied in similar manner using 4-fiber rings. Thetoken circulates in one direction only, but packets can-be transmittedin both directions.

FIG. 16 shows another example of the configuration of a node. In thisexample, path setup section 23 for setting up a path prior totransmission of control signals is added to the node configuration shownin FIG. 4.

FIG. 17 to FIG. 21 show the control flow of path setup at each type ofnode. FIG. 17 shows the overall control flow, FIG. 18 shows theoperation of a source node, FIG. 19 shows that of an intermediate node,FIG. 20 that of a bridge node, and FIG. 21 that of a destination node.

Each node performs the control shown in FIG. 18 as a source node (S2)when there is a path connection request (S1). Likewise, each nodeperforms the control shown in FIG. 19 as an intermediate node (S9) whenit has received a “connection request packet” from another node (S3) andit is necessary to operate as an intermediate node (S8). Again, eachnode performs the control shown in FIG. 20 as a bridge node (S7) when ithas received a “connection request packet” from another node (S3) and itis necessary to operate as a bridge node (S6). Finally, each nodeperforms the control shown in FIG. 21 as a destination node (S5) when itreceives a “connection request packet” from another node (S3) and thenode in question has been designated the destination node (S4).

Although FIG. 17 to FIG. 21 show the control performed at a single node,when a path is being set up, a plurality of related nodes operaterespectively as a source node, intermediate nodes, bridge nodes and adestination node. These operations will now be explained, using as anexample the case of setting up a path between two ring networksincluding ring #a and ring #b.

When a node at which there is a path setup request (the source node)seizes a token (S11) which is circulating in one direction only in ring#a, it writes the path grade, capacity and so forth in a connectionrequest packet and transmits this packet in both directions, i.e.clockwise and counter-clockwise, as a routing packet, to perform adouble route search (S12). Each intermediate node stamps its ownidentifier (ID) in the repeater node area of a received routing packetif there is sufficient spare bandwidth to transfer the relevant path inthe direction the received packet is traveling (S21, 23), and thenrepeats that packet. If the path cannot be transferred to the next node,the intermediate node writes “no good” (NG) (S22). When a bridge nodereceives the routing packet, it stamps ID or NG in the relevant area ofthe packet in accordance with whether or not it can connect the path tothe next ring #b, duplicates the packet and stores the duplicate inbuffer memory, rewrites the C field of the original packet, and sendsthis packet into ring #a as a “response packet” (S31, S32, S34). Aresponse packet which has returned to the source node after travellingaround ring #a is terminated (S13) and the token is released (S14). Atoken can be released after a packet is terminated, or after the tokenis terminated. Alternatively, it can be released after several packetshave been transmitted, or a multi-token control scheme can be adopted.Here, for simplicity, a single token and packet termination scheme willbe explained.

Because a packet is transferred both clockwise and counter-clockwise,two nodes in ring #a constitute bridge nodes. Each of these two bridgenodes seizes a token in ring #b and transmits the stored “routingpacket” towards the destination node in the same direction as thereceived packet flow (S33, S35). A bridge node achieves class A pathroute diversity by transmitting a packet received in the clockwisedirection from ring #a in the clockwise direction in ring #b, and bytransmitting a packet received in the counter-clockwise direction fromring #a in the counter-clockwise direction in ring #b. The samepreservation of direction applies to routing within ring #b as well.

After the destination node in ring #b has confirmed that the path can beconnected (S41), it transmits a “routing completion” notification packet(S46). If the path cannot be corrected, it transmits a “routinginformation packet” by writing NG. If the path of interest requiresclass A, two “routing information packets” are transmitted via the tworoutes (S45, S49). If the path requires class B or C, a single “routinginformation packet” is sent via the shorter route (S44, S48). In otherwords, for class B and class C paths, the routing completionnotification packet is transferred via one route only from thedestination node to the source node. Note that at the same time as eachbridge node transfers a “routing information packet” to the next ring,it uses the opposite route to transmit the response packet in ring #bupon receipt of the “routing information packet” (S36, S37).

For a class B path, each intermediate node on the opposite side of thering from the working route watches the content of this “responsepacket”, and if it can guarantee a protection path within the ring,writes its ID in the “response packet” (S21, S23). If it cannotguarantee a protection path, it writes NG (S22). The NG flag issubsequently transferred to the source node from the bridge node ordestination node which terminates the “response packet”.

When the “response packet” is received, the source node releases thetoken (S14) and sets up the requested path after the routing informationhas been received (S15). If the routing information cannot be received Ntimes (S15, S16), routing is respected.

The average delay from the occurrence of a path connection request toconnection establishment depends on the frequency of the connectionrequests. If the occurrence of connection requests is assumed to be arandom process, average delay can be expressed by the followingequation: $\begin{matrix}{T = {\left( {{\frac{1}{2\left( {\rho - 1} \right)}\quad \left( {\rho + 1 - \frac{\rho}{N}} \right)} + \frac{3}{2}} \right)*N*\left( {{\tau_{f}*\frac{L_{h} + {\left( {N + 2} \right){\log \left( {{n\quad*N} + 1} \right)}}}{d}} + {t\quad L} + \tau_{p}} \right)}} & (2)\end{matrix}$

where N is the number of nodes in a ring, τ_(f) is the transfer time forone SDH frame, d is the number of DCC channel bits per frame, n is thenumber of rings, t is the propagation delay, L is the link distance,τ_(p) is the packet processing time per node, L_(h) is the packetoverhead, and p is connection request frequency.

FIG. 22 shows the relation between path connection request frequency andaverage connection delay obtained by means of Equation 2. The horizontalaxis plots connection request frequency ρ, which shows how manyconnection requests occur during a round trip of the token in a ring. Itis assumed that the total number of nodes is 50, τ_(f)=125 μs, d=12bytes, t=5 ns/m, L=100 km for each link, τ_(p)=1 ms, and L_(h)=8 bytes.Average connection delay decreases with increasing number of rings(i.e., with decreasing number of nodes in a ring). This is because theprobability of a given node getting the token increases due to the totalnumber of tokens increasing. It will be seen that if the total number ofnodes is about 50, the average connection delay is of the order ofseconds.

An explanation will now be given of self-healing.

A class A path is a high-reliability path which never loses even asingle bit of information. This is achieved by duplicating the path atthe source node and accommodating it in two different routes, and byhitless switching at the destination node.

FIG. 23 shows an example of a configuration for hitless switching. Theclass A path passes from two TTFs (Transport Terminating Function) atthe source node side through separate paths (path 0 and path 1), and isterminated by two TTFs at the destination node side. The delay in path 0is τ₀ and the delay in path 1 is τ₁. The outputs of the two transportterminating functions at the destination node side are connected tohitless switch SW via delays DLY which can be based on memory, and theamount of delay introduced by delays DLY, and the operation of hitlessswitch SW, are controlled by switching control SWC. Switching controlSWC compares the two paths frame-by-frame and activates the switch whenit detects an error in the B3 byte of a VC frame. When an error isdetected, the switch selects the better quality path. The effect of thisscheme is not only hitless switching in the event of an unforeseenfailure, but also an improvement in transmission performance undernormal conditions, with the output BER becoming lower than the BER ofthe transmitted path. The output BER ε^(out) of the switch can beexpressed by the following equation:

Q(ε^(out))=Q(ε⁰)*Q(ε¹)  (3)

where ε¹ is the BER in the working path and ε² is the BER in theprotection path. Q(ε) is the probability of an error being detected inone frame on the basis of B3 (BIP-8), and can be expressed as follows:

Q(ε)=1−|1−P(ε)|⁸  (4)

where P(ε) is the probability of detecting a parity error in one rail ofBIP-8, and can be expressed as follows:

P(ε)=1−^(1+(1−2ε)) ^(N) ^(R)  (5)

FIG. 24 shows the error improvement effect obtained when the path isassumed to be a VC-4 (156 Mbit/s). If input BER is 10⁻¹⁰ on both theworking and protection paths, output BER at the hitless switchingcircuit can be improved to as low as 10⁻¹⁶.

Next, an explanation will be given of self-healing for class B paths.Class B paths are ranked as medium reliability paths which, althoughthey do not require hitless switching, do require switching to aprotection route in the event of a failure in the network. Whereas pathrestoration for a class A path involves dedicated switching, a class Bpath employs shared switching. In conventional shared switching schemes,completely different methods and equipment have been used for path setupand path restoration. As opposed to this, in the present invention thesame algorithm that is employed in path setup is used for pathrestoration as well, and some equipment is shared. Below, an explanationwill be given of the differences between path restoration and pathsetting.

When a failure occurs (e.g., a link failure), the nodes at either end ofthe failed link detect Loss of Signal (LOS) and generate a sectionalarm. Whereas in the normal state the token was transferred around thering in one direction only, when the failure occurs the two adjacentnodes loop back the token on the section overhead of opposite direction(the function can be realized in the firmware or software platform),causing it to be transferred around the ring in opposite directions. Bydoing this, all the nodes can get the token despite the link failure. Animportant point is that it is only the token contained in the protocol(the FC bytes) which is looped back, whereas the main signal is restoredin the path layer. In the failure state, all the class B paths that hadbeen carried by the failed link request path connection, and unlike theoccurrence of connection requests at path setup, this is not a randomprocess. A node which has seized the circulating token reconnects asingle failed path relating to that node using the same algorithm as inthe case of path setup, and releases the token. The node which nextseizes the token does the same. In this way, the paths are successivelyreconnected one by one.

FIG. 25 to FIG. 27 serve to explain the self-healing characteristics ofclass B paths. FIG. 25 shows the situation where a failure has occurredin the j-th ring network containing nodes A, B, C, D, and Z. FIG. 26shows the relation between node position and the number of paths whichhave to be restored, on the assumption that all the failed paths areclass B. FIG. 27 shows schematically how a token is transferred betweenall the nodes by loop-back.

It will be assumed that a worst-case failure has occurred in the ring,namely, in the link between bridge node A and its adjacent node Z. Pathsaffected by this failure include inter-ring paths #1 which pass throughthis j-th ring network (equivalent to the second term of Equation 1);inter-ring paths #2 which connect this j-th ring network to another ringnetwork (equivalent to the third term in Equation 1); and intra-ringpaths (defined by the first term in Equation 1). The restoration ofinter-ring paths #1 has to be shared by nodes A and D which are bridgenodes of the j−1^(th) and the j+1^(th) ring networks, respectively. Theparameter m in FIG. 26 is the average apportionment coefficient betweennodes A and D (note that processing based on token protocol averages theburdens of path reconnection processing), and can be expressed by thefollowing equation: $\begin{matrix}{m = {- \frac{8 - j - {3N} + {3j\quad N} - {3n} + {j\quad n} + {N\quad n} - {j\quad n\quad N}}{2\left( {j - 1} \right)\left( {n - 3} \right)\left( {N - 1} \right)}}} & (6)\end{matrix}$

Because nodes A and Z are located at the ends of the failed link, thetoken is looped back at these nodes A and Z. At any given node, theprocessing time for simply transferring a packet to a neighboring nodewill be termed τ_(r1), and the processing time for restoring a path willbe termed τ_(r2). These two parameters can be expressed as follows:$\begin{matrix}{{\tau_{r1} = {{\tau_{f}\quad \frac{L_{h} + {\left( {N + 2} \right){\log \left( {{n\quad N} + 1} \right)}}}{d}} + {t\quad L} + \tau_{p}}}{\tau_{r2} = {N\quad \tau_{r1}}}} & (7)\end{matrix}$

Restoration performance of class B paths at a given time can becharacterised as shown in the following table: namely, as a function ofk, the number of times a token loops back.

Loops-back times Time Restored paths 1 < k < (N − 1)/2 Nτ_(r1) + (N −k + 1)τ_(r2) (N − k + 1)τ_(r2) k < max[n − j,j − 1](N − 1) Nτ_(r1) +½(N + 1)τ_(r2) ½(N + 1)τ_(r2) k < (N − 1)/2 + (j − 1)(N − 1) Nτ_(r1) +(3 + (N − 1)/2 − k)τ_(r2) (3 + (N − 1)/2 − k)τ_(r2) k < (N − 1)/2 + (j −1)(N − 1) + Nτ_(r1) + 2τ_(r2) 2τ_(r2) m/2(n − j)(j − 1)(N − 1)²

The restoration ratio R(T) is defined as the ratio of the number ofrestored paths at a given time to the total number of failed paths, andcan be expressed as follows: $\begin{matrix}{{\text{~~~~}{R(T)}} = {{R_{i}(T)} + {\sum\limits_{h = 1}^{i - 1}{R_{h}\left( {\Delta t}_{h} \right)}}}} & (8) \\\text{where:} & \quad \\{{\text{~~~~}{\sum\limits_{h = 1}^{i - 1}{\Delta t}_{h}}} < T < {\sum\limits_{h = 1}^{i}{\Delta t}_{h}}} & (9) \\{{\text{~~~~}{R_{1}(t)}} = {{\sum\limits_{h = 1}^{s{(t)}}\frac{N - h}{A}} + {\frac{1}{A}\quad \frac{{N\quad s(t)} - 1}{{N\quad t_{r1}} + {\left( {N - {s(t)} - 1} \right)\quad t_{r2}}}\quad t}}} & (10) \\{{\text{~~~~}{R_{2}(t)}} = {\frac{1}{A}\quad \frac{N + 3}{{2N\quad t_{r1}} + {\left( {N + 3} \right)\quad t_{r2}}}\quad t}} & \quad \\{{\text{~~~~}{R_{3}(t)}} = {{\sum\limits_{h = 1}^{k{(t)}}\frac{N + 5 - {2h}}{2A}} + {\frac{1}{A}\quad \frac{N + 5 - {2k\quad (t)}}{{2N\quad t_{r1}} + {\left( {N + 5 - {2{k(t)}}} \right)\quad t_{r2}}}\quad t}}} & \quad \\{{\text{~~~~}{R_{4}(t)}} = {\frac{1}{A}\quad \frac{2}{{N\quad t_{r1}} + {2\quad t_{r2}}}\quad t}} & \quad \\\text{and:} & \quad \\{{\text{~~~~~~~}{s(t)}} = \left\lbrack {{N\quad \frac{t_{r1}}{t_{r2}}} + \left( {N + \frac{1}{2}} \right) - \sqrt{\left( {\left\{ {{N\quad t_{r1}} + {\left( {N + \frac{1}{2}} \right)\quad t_{r2}}} \right\}^{2} - {2t_{r2}*t}} \right)}} \right\rbrack} & (11) \\{{\text{~~~~}{k(t)}} = \left\lbrack {{N\quad \frac{t_{r1}}{t_{r2}}} + \left( {\frac{N}{2} + 3} \right) - \sqrt{\left( {\left\{ {{N\quad t_{r1}} + {\left( {\frac{N}{2} + 3} \right)\quad t_{r2}}} \right\}^{2} - {2t_{r2}*t}} \right)}} \right\rbrack} & \quad \\{{\text{~~~~}{\Delta t}_{1}} = {\frac{N - 1}{2}\quad \left( {{N\quad t_{r1}} + {\frac{3}{4}\quad \left( {N + 1} \right)\quad t_{r2}}} \right)}} & (12) \\{{\text{~~~~}{\Delta t}_{2}} = {{\left( {{N\quad t_{r1}} + {\frac{N + 1}{2}\quad t_{r2}}} \right)\quad \left( {n - j} \right)\quad \left( {j - 1} \right)} - \frac{N - 1}{2}}} & \quad \\{{\text{~~~~}{\Delta t}_{3}} = {\left( {{N\quad t_{r1}} + {\frac{N + 1}{2}\quad t_{r2}}} \right)\quad \frac{N - 1}{2}}} & \quad \\{{\text{~~~~}{\Delta t}_{4}} = {\left( {{N\quad t_{r1}} + {2\quad t_{r2}}} \right)\quad \frac{m}{2}\quad \left( {n - j} \right)\quad \left( {j - 1} \right)\quad \left( {N - 1} \right)^{2}}} & \quad\end{matrix}$

FIG. 28 shows restoration ratios calculated using the above equations.The total number of nodes is assumed to be 50 and calculation resultsare given for the case of a single ring, two rings, and five rings. Therest of the parameters are the same as in FIG. 22. FIG. 28 also showsthe restoration ratios for class A, where hitless switching achievesinstantaneous restoration, and for class C, where path restoration hasto wait for hardware maintenance. For class B paths, completerestoration takes more than 40 seconds in the case of a single ring, is20 seconds with two rings, and is around 10 seconds when there are fiverings. It is anticipated that restoration time will be longer with thefirst embodiment than with the second embodiment. The reason for this isthat in the first embodiment the number of packets to be processedincreases in proportion to the number of nodes.

An important point in the second embodiment is that with a multiple-ringconfiguration, path setup can be performed without information onnetwork configuration. Moreover, modifying this method would enableinformation on the network configuration of node deployment.

Explanations will now be given of how methods for obtaining informationon network configuration of node deployment and for recognizing nodecondition can be applied to the operation and management of distributedcontrol type multiple-ring networks.

Japanese Laid-Open Patents No. 8-191318 and No. 7-58765 disclose methodsfor recognising conditions in a single ring with centralised control.These methods require a master node for gathering information andcontrolling the network. Japanese Laid-Open Patent No. 8-23200 disclosesa condition recognition method which is applicable to distributedcontrol. According to this method, a remote device which can beconnected to an arbitrary node learns node conditions by causing amanagement packet to circulate on the loop, this management packethaving a pointer indicating node number and a region in which an addresscan be set. Given that this system enables control to be carried outfrom an arbitrary node, it can be implemented in a distributed controlsystem as well.

However, if this system is implemented without modification on amultiple-ring network, the two bridge nodes which connect a given tworings will each independently transfer a management packet to the nextring. Consequently, if the number of rings is N, the number ofmanagement packets in the network as a whole will be 2^(N), therebygiving rise to the problem that the “management packets” causescongestion which results in degradation of network performance.

An explanation will now be given of an embodiment which solves suchproblems and which makes it possible to learn how nodes are arranged andto recognize their condition in distributed control multiple-ringnetworks in which each pair of ring networks is connected by two bridgenodes.

FIG. 29 shows a third embodiment of the present invention. Thisembodiment is a multiple-ring transmission facility which has six ringnetworks 61 to 66, each comprising a plurality of nodes connected inring form by transmission lines. Ring networks 61 and 62 are connectedvia bridge nodes 71 and 72, ring networks 62 and 63 are connected viabridge nodes 73 and 74, ring networks 61 and 64 are connected via bridgenodes 75 and 76, ring networks 62 and 65 are connected via bridge nodes77 and 78, ring networks 63 and 66 are connected via bridge nodes 79 and80, ring networks 64 and 65 are connected via bridge nodes 81 and 82,and ring networks 65 and 66 are connected via bridge nodes 83 and 84.Ring networks 61 to 66 have nodes other than bridge nodes, but in FIG.29 these other nodes are omitted with the exception of source node 91which transmits “node information collecting packets” for collectinginformation relating to the network configuration of node deployment andcondition of nodes. FIG. 29 also shows an example of a transfer routefor a node information collecting packet.

In this embodiment, connection between nodes of ring networks 61 to 66is performed under distributed control, and this control is achieved byefficiently collecting information relating to the network configurationof node deployment and conditions of the nodes in each ring. Thisinformation can be collected from any node. In the followingexplanation, it is assumed that it is source node 91 in ring network 61which is collecting information. Transmission of a node informationcollecting packet from source node 91 takes place either when thenetwork is built, or when a new node is added. In other words, the nodewhich constitutes source node 91 can be a node at which actions forsetting up the network are performed when the network is built, or itcan be a node which is added once the network is operating.

First of all, using a token protocol constructed on the section overhead(SOH) in similar manner to the second embodiment, source node 91transmits a node information collecting packet in one direction aroundring network 61. The signal for collecting node information is carriedin the data field section of the node information collecting packet.Only one token is present in a ring network, and only a node that hasseized the token can transmit a packet.

A node information collecting packet that has been sent from source node91 circulates around ring network 61 on the SOH. Each node successivelystamps its ID in the packet, or writes NG if the node has failed. Apacket which has travelled around the ring and returned to source node91 is stored in memory and terminated.

The pair of bridges which connect ring networks 61 and 62, i.e., bridgenodes 71 and 72, can store the node information collecting packet inmemory so that it can be passed over to the next ring network 62. Ofthese two bridge nodes 71 and 72, only bridge node 71, which seized thefirst token, transfers the node information collecting packet to thenext ring network 62. Bridge node 72, which has received the nodeinformation collecting packet transferred by the other bridge node 71but which has not seized the token, deletes the packet in its memory.

Bridge node 71, which has transferred the node information collectingpacket, stores the node information collecting packet which hascirculated around the ring and returned, terminates it, and sends itback to original ring network 61. At this point, it inhibits the othernodes to write any information on the packet, which is possible bychanging C field in the packet.

If a node which has already processed a packet receives the same packetagain within a prescribed time, it deletes that packet. For simplicity,FIG. 29 shows the deletion of packets by bridge nodes, but this can becarried out at any node.

By repeatedly forwarding and returning “node information collectingpackets” between ring networks, one node information collecting packetis returned from each of the ring networks 61 to 66, and information onthe network configuration of node deployment and condition of all thenodes can be efficiently collected.

FIG. 30 to FIG. 33 show the processing flow, at each type of node, forcollecting and recognizing information relating to network configurationof node deployment and node condition. FIG. 30 shows the processing flowat a source node which wants to collect information relating to networkconfiguration of node deployment and condition of the nodes. FIG. 31shows the processing flow at an intermediate node, FIG. 32 shows theprocessing flow at a bridge node, and FIG. 33 shows the packetprocessing flow in the processing shown in FIG. 32. These processingsare performed by means of path data collector 10 and controller 21 inthe configuration shown in FIG. 4.

When a source node seizes the token circulating within the ring to whichthe node belongs (S51), it transmits a “node information collectingpacket” (S52). This node information collecting packet has, in its datafield section which is used for a conventional token ring protocol, anarea in which each node can place its stamp. The address of thedestination node is set as the source node address. After the sourcenode has sent a “node information collecting packet”, it stores all thepackets it receives (S55). Note that when it receives the first packetafter this has circulated around the ring, the node releases the token(S54).

Nodes other than the source node operate as intermediate nodes when theyreceive a “node information collecting packet” (S61). First of all, theycheck whether or not the received packet is write-inhibited (S62). If itis, the packet is transferred to an adjacent node. If the receivedpacket is the same as a previously received one (S63), it is deleted(S64). If the packet has not been received before and the node receivingthe packet is normal, the node writes its own ID in the stamp area andsends the packet on. If the packet has not been received before and afailure has occurred somewhere in the node receiving the packet, thenode writes NG in the stamp area, and sends the packet on. The ID of anode contains a ring number as well as the actual node number. Forexample, four bits of the byte can be used as the ring number and theother four bits can be used as the number of the node within that ring.

The processing performed by a bridge node can be broadly divided intothree types. The first is processing as an intermediate node (S71). Thesecond is the processing of “node information collecting packets” as abridge (S72). The third is the processing of “ring configurationinformation packets” (S73). A “ring configuration information packet” isa write-inhibited information notification packet. In FIG. 33, “nodeinformation collecting packets” and “ring configuration informationpackets” are collectively termed “X packets”.

After a bridge node has performed the processing required by anintermediate node, it enters the mode for processing “node informationcollecting packets”. If it receives a “node information collectingpacket” from a source node (S81), it stores the packet in memory (S82)so that it can be passed on to the next ring. It then enters the modefor waiting for a token of the next ring (S83). When it secures a token,it transmits the “node information collecting packet” (S87). If itreceives the same “node information collecting packet” from anotherbridge before securing a token (S84), it immediately deletes the packetstored in its memory (S85) and enters the processing mode of anintermediate node (S86). As a result, because only one “node informationcollecting packet” is transmitted from each bridge in a ring, congestionby control packets can be avoided. Next, the bridge node enters the modefor receiving packets which it itself has previously forwarded (andwhich are now filled with information). When it receives such packets(S89, S90), it write-inhibits them (S91) and returns them all to theoriginal ring by the same route. If it receives a packet that hasalready been write-inhibited (S88), it returns that packet to the ringwhich transmitted it, whereupon the processing is finished.

By means of these operations, a source node can collect and recognizeinformation on the network configuration of node deployment andcondition of all nodes. After all this information has been stored(S53-S55) (and if processing time is greater than T (S56)) the sourcenode sends out “ring configuration information packets” in a similarmanner (S57, S58). The intermediate nodes receive these and any packetwhich has previously been received is deleted (S63, S64). Note that the“ring configuration information packets” are write-inhibited. Bridgenodes transfer these packets in a similar manner and store theinformation contained in them. Intermediate nodes and bridge nodes bothfinish the node information collecting process at the stage at whichthey have received and forwarded a “ring configuration informationpacket”. The source node finishes the node information collectingprocess and releases the token at the stage at which it has received the“ring configuration information” (S59, S60).

The optimum path between any two nodes is set up on the basis of nodeinformation collected in this manner.

FIG. 34 shows an example in which a path has been set up in themultiple-ring transmission facility shown in FIG. 29. In the exampleshown here, a double path has been set up between source node 92 in ringnetwork 61 and destination node 93 in ring network 63. (Note that theterm “source node” used here means a transmitting source at path setup,and is different from a source node for purposes of node informationcollection, as described above.) Some of the references cited aboveconsidered the case where double paths are routed through the same ringnetworks, but no consideration was given to a case where there is apossibility of setting up paths which are routed through different ringnetworks. Accordingly, FIG. 34 illustrates a case where, in themultiple-ring transmission facility shown in FIG. 29, one of the routesof the double path from source node 92 of ring network 61 to destinationnode 93 of ring network 63 is set up via ring network 62, while theother is set up via ring networks 64, 65 and 66. This situation resultsin large differences in the delay between the two routes. This isbecause it is always the bridge node which a packet first encounterswhich transfers the path to the next ring network. When there is a largedifference in delay between two routes, a large buffer will be requiredto ensure that the path can be switched hitlessly.

To solve this problem, restrictions have to be placed on which ringseach bridge node can transfer the path to. In other words, the set ofring networks through which paths from ring network 61 to ring network63 pass has to be uniquely determined, and each bridge node has to getthis information.

Information which has been collected by “node information collectingpackets” should therefore be distributed by “ring configurationinformation packets” from source node 91, which constituted thetransmitting source of the “node information collecting packets”, tobridge nodes 71 to 84 of ring networks 61 to 66, and if required, toeach node. On the basis of this distributed information, each bridgenode 71 to 84 places a restriction on path setup by way of itself. Inthe example shown in FIG. 34, bridge node 75 places a restriction onpath setup between source node 92 and destination node 93. As a result,if a class A path is to be set up between source node 92 and destinationnode 93, the path for the second route can be set up via ring network62, thereby obtaining a double path with little delay difference. Thisenables the size of the buffer required in destination node 93 toperform hitless switching to be reduced.

FIG. 35 shows the processing flow for routing by a bridge node. Bridgenodes first of all perform routing under any restriction that has beenimposed (S101). For example, although the routes through which a class Apath passes are completely different, it is arranged for the ringnetworks through which they pass to be exactly the same, and for theroutes joining the ring networks to be uniquely determined. The routedecision is simple because the number of rings is much smaller than thenumber of nodes. If path setup under this sort of restriction ispossible (S102), this completes the routing of the paths. In this caseit is easy to achieve hitless switching and a path of the highestreliability is obtained. If path setup under restriction is impossible,the restriction is removed and re-routing carried out (S103). In thiscase, although it is difficult to achieve a hitlessly switchable path, ahigh reliability path can still be obtained. On the whole, therefore,buffer size can be reduced.

As has been explained above, the present invention provides a trunktransmission network which ensures that important information can alwaysbe successfully transferred to its destination node, and thatinformation where the main concern is economy can be sent very cheaply.This is achieved by providing just three classes of path between nodesof interest, namely: class A paths which involve setting up a firstroute and a second completely diverse route so that in the event of afailure, the information in the path can be hitlessly switched; class Bpaths in which, in the event of a failure, path information flow isrestored by searching for a new route for the path; and class C pathswhere information in the path at the time of a failure is not recovered,and any subsequent restoration depends on repair of equipment.

Furthermore, by managing paths between nodes using only three classes ofhigh-capacity path, the number of paths to be managed in a trunktransmission network can be reduced and the burden on the operatingsystem can be eased.

In addition, high reliability and survivability with easy pathrestoration in the event of a failure can be guaranteed for a trunktransmission network by connecting a plurality of nodes in a ring,arranging a plurality of these ring-shaped networks in a plane, andconnecting them to each other at two or more nodes. Moreover, thefollowing effects are obtained by basing a trunk transmission network ona ring topology, namely: the direction of a path is restricted to beingeither clockwise or counter-clockwise; routing for path setup andre-routing for path restoration when a failure has occurred can besimplified; the scale of the hardware and complexity of the algorithmsrequired for route compilation can be reduced; and an economical trunktransmission network can be constructed.

Previously, a centralized operating system and connection equipment forconnecting each node to this operating system were required. With thepresent invention, however, these can be dispensed with by providingeach node with control means for control communication directly withother nodes. Then, when a path is being set up, the node at which thepath setup request has occurred successively selects and sets a route tothe destination node by accessing each node directly and confirmingwhether it has spare capacity, and not by using a centralized operatingsystem. In addition, when setting up the path, the node selects theroute while finding out whether a path is already accommodated at theother nodes. The previous technology, in addition, obviates the need fora huge database for designing where paths could be accomodated. It alsoobviates the need for a route selection tool based on this hugedatabase. The present invention therefore provides an economical trunktransmission network.

The present invention also provides a more economical trunk transmissionnetwork by enabling the load on the operating system to be reduced. Thisis achieved by the following means. Firstly, the routing algorithmdescribed above is used to route a diversely routed first route andsecond route, which together comprise a class A path, around a ringseparately and in such manner that one is routed clockwise and the otheris routed counter-clockwise, and at the two nodes which inter-connectrings, the clockwise route is connected in the clockwise direction andthe counter-clockwise route is connected in the counter-clockwisedirection, so that the first and second routes do not intersect.Secondly, when a failure has occurred on a class B path, the path isrestored by using the routing algorithm described above to set the pathup again in a route which avoids the failed link or node. In otherwords, instead of separately designing a routing algorithm for use innormal conditions and a path recovery algorithm for self-healing, as inthe prior art, the routing algorithm can be used both for normalconditions and for path restoration after a failure.

The present invention is therefore capable of setting up paths with aplurality of transport functions. Namely, for information requiringhigh-reliability transmission, it can set up a path capable oftransmitting the information without dropping any bits in the event of afailure, while information that does not require such reliability can betransmitted economically. The present invention also makes it easy toconstruct a ring topology network capable of guaranteeing highreliability and survivability.

Automatic network operation invariably requires learning the networkconfiguration of node deployment and conditions of network nodes, and inthe present invention this is implemented under distributed control. Asa result, the present invention can provide simple and high-speednetwork management, node condition management, and route diversity withlittle delay variation, and obviates the need to have a plurality ofcontrol packets, which has previously been a potential problem indistributed control.

The foregoing describes specific representative embodimentsmodifications of which will occur to those of ordinary skill in the artand which are included in the scope of the invention defined by thefollowing claims.

What is claimed is:
 1. A trunk transmission network comprising: aplurality of nodes connected via physical transmission lines; aplurality of paths for transmitting information signals being set up onsaid physical transmission lines between said nodes; for an informationsignal which is to be transmitted from one of the plurality of nodestermed a source node to another of the plurality of nodes termed adestination node, each path connects the source node and the destinationnode either directly or via other nodes; said paths which are betweensource and destination node pairs being set up on the basis ofpre-classification into higher service class paths in which any loss ofinformation occurring in that path is restored, and lower service classpaths which permit loss of information to occur in the path; and eachnode including means which, when that node is a source node, recognizesthe service class of the information signal to be sent to thedestination node and sets up a path corresponding to that service classto thereby provide distributed control of path setup.
 2. A trunktransmission network according to claim 1, wherein: the higher serviceclass is further divided into a highest class and a middle class; saidhighest class path employs complete diversity routing wherein aplurality of different routes are set up for that path; said middleclass path can be re-routed around the location of a failure when saidfailure has occurred in a portion of the route traversed by the path;and said lower service class path is not alternatively routed when afailure has occurred in the route traversed by the path.
 3. A trunktransmission network comprising: a plurality of nodes connected viaphysical transmission lines; a plurality of paths for transmittinginformation signals being set up on said physical transmission linesbetween said nodes; for an information signal which is to be transmittedfrom one of the plurality of nodes termed a source node to another ofthe plurality of nodes termed a destination node, each path connects thesource node and the destination node either directly or via other nodes;said paths which are between source and destination node pairs being setup on the basis of pre-classification into higher service class paths inwhich any loss of information occurring in that path is restored, andlower service class paths which permit loss of information to occur inthe path; and each node including means which, when that node is asource node, recognizes the service class of the information signal tobe sent to the destination node and sets up a path corresponding to thatservice class; wherein each of at least some of the plurality of nodeshas a distributed path setup means which sets up paths prior totransmission of an information signal by using a control channel toexchange control signals with other nodes to thereby provide distributedcontrol of path set up.
 4. A trunk transmission network according toclaim 3, wherein the distributed path setup means selects a route, inaccordance with a required service class, from among the plurality ofroutes which can connect the source and destination nodes, and thensuccessively sets up a path along the selected route, starting from thesource node.
 5. A trunk transmission network according to claim 3,wherein: said higher service class is further divided into a highestclass and a middle class; the physical transmission lines are in theform of a plurality of ring networks connected together, each ringnetwork comprising at least two nodes connected in a ring; each saidring network is connected to another ring network by means of some ofnetwork nodes acting as bridge nodes; and the distributed path setupmeans includes means which, for the highest class path, sets up twopaths in mutually opposite directions around each said ring networkthrough which the highest class path passes; and which, for the middleclass path and the lower service class path, sets up a path in onedirection around each said ring network.
 6. A trunk transmission networkaccording to claim 5, wherein each node has means for hitlesslyselecting the better quality route of the highest quality pathcomprising two routes for which that node is the destination node.
 7. Atrunk transmission network according to claim 5, wherein the path setupmeans includes means for automatically restoring the middle serviceclass path by re-routing in the event of a failure.
 8. A trunktransmission network according to claim 7, wherein the restoration meansincludes means for looping back a token contained in the control channelwhen a node has detected a failure in an adjacent link or node.
 9. Atrunk transmission network according to claim 5, wherein: each said ringnetwork is connected to another ring network via at least two bridgenodes; at least one node of any of the ring networks comprises means fortransmitting, in one direction around the ring network to which thatnode belongs, a node information collecting packet for collectinginformation relating to the network configuration of node deployment andoperating state of the nodes in that ring network and in another of saidring networks; and means which terminates a node information collectingpacket which has returned to that node which originally transmitted it,and which stores the information collected by that node informationcollecting packet; each node of each ring network comprises means whichwrites its node ID and state in a received node information collectingpacket and transfers the packet to the next node; and each node used asa bridge node comprises, in addition to the node information collectingpacket writing and transfer means; means for temporarily storing a nodeinformation collecting packet received from one of two ring networksmutually connected by the bridge node; means which, when a right totransmit to another of the two ring networks has been received,transfers to said another ring network the node information collectingpacket stored in the temporary storage means; means which deletes thenode information collecting packet stored in the temporary storage meansif a node information collecting packet from another bridge node hasbeen received but no transmitting right has been obtained; and means forterminating a node information collecting packet which has returned tothe bridge node which originally transferred it, and for writeinhibiting that node information collecting packet and returning it tothe original ring network.
 10. A trunk transmission network according toclaim 9, wherein each said node of each said ring network, including thebridge nodes, comprises means which, if it receives the same nodeinformation collecting packet within a predefined time, deletes the samenode information collecting packet.
 11. A trunk transmission networkcomprising: a plurality of nodes connected via physical transmissionlines; a plurality of paths for transmitting information signals beingset up on said physical transmission lines between said nodes; for aninformation signal which is to be transmitted from one of the pluralityof nodes termed a source node to another of the plurality of nodestermed a destination node, each path connects the source node and thedestination node either directly or via other nodes; said paths whichare between source and destination node pairs being set up on the basisof pre-classification into higher service class paths in which any lossof information occurring in that path is restored, and lower serviceclass paths which permit loss of information to occur in the path; andeach said node including means which, when that node is a source node,recognizes the service class of the information signal to be sent to thedestination node and sets up a path corresponding to that service class;wherein: the higher service class is further divided into a highestclass and a middle class; a highest class path employs completediversity routing wherein a plurality of different routes are set up forthat path; a middle class path can be re-routed around the location of afailure when said failure has occurred in a portion of the routetraversed by the path; said lower service class path is notalternatively routed when a failure has occurred in the route traversedby the path, said higher service class is further divided into a highestclass and a middle class; the physical transmission lines are in theform of a plurality of ring networks connected together, each ringnetwork comprising at least two nodes connected in a each said ringnetwork is connected to another ring network by means of some of networknodes acting as bridge nodes; and the distributed path setup meansincludes means which, for the highest class path, sets up two paths inmutually opposite directions around each said ring network through whichthe highest class path passes; and which, for the middle class path andthe lower service class path, sets up a path in one direction aroundeach said ring network; wherein distributed path setup means comprises:means which, when the node is a source node, gets a token that iscirculating around the ring network to which that node belongs, and thensends path setup request packets in two mutually opposite directions;means which, when the node is a bridge node, transfers the path setuprequest packet that has arrived in one direction to another ring networkin the same direction; and means which, when the node is a destinationnode and the packets received from the two directions request setup ofthe highest class path, sends back a response to these packets in twomutually opposite directions, and when the packets received from the twodirections request setup of the middle class path or the lower serviceclass path, sends back a response to one of these packets in onedirection only.
 12. A trunk transmission network according to claim 11,wherein each node has means for hitlessly selecting the better qualityroute of the highest quality path comprising two routes for which thatnode is the destination node.
 13. A trunk transmission network accordingto claim 11, wherein the path setup means includes means forautomatically restoring the middle service class path by re-routing inthe event of a failure.
 14. A trunk transmission network according toclaim 13, wherein the restoration means includes means for looping backa token contained in the control channel when a node has detected afailure in an adjacent link or node.
 15. A node for transmittinginformation signals via paths set up in advance in physical transmissionlines in a trunk transmission network, comprising: means which, for thetransmission of an information signal from that node as a source node,recognizes whether a service class of the information signal to betransmitted is a higher service class for which any loss of informationin one of the paths is restored, or a lower service class which permitsloss of information in one of the paths, and sets up a pathcorresponding to that service class.
 16. A node according to claim 15,wherein the means for selecting a path includes means which furtherdivides the higher service class into a highest class and a middleclass; and which, for an information signal of the highest class,selects a path comprising a double route; and, for an information signalof the middle class, selects a path in one direction which can detouraround the location of a failure when a failure has occurred in saidportion of the route; and, for an information signal of a lower serviceclass, selects a path in one direction, which is not alternativelyrouted when a failure occurs in that route.
 17. A node according toclaim 15, wherein said node includes a distributed path setup meanswhich sets up said paths prior to transmission of an information signalby using a control channel to exchange control signals with other nodes.